1. Field of Invention
The present invention relates to an impedance match circuit. More particularly, the present invention relates to an impedance match circuit having high precision without being affected by variations in manufacturing processes.
2. Description of Related Art
The impedance match circuit is applied in various fields. For example, when high-speed data are transmitted through a transmission line, an extremely high frequency results in a very short wavelength, thus generating a significant electromagnetic wave effect. If the input unit impedance, the transmission line characteristic impedance, and the output unit impedance are not matched, the process of data transmission will generate a reflection interference phenomenon due to an electromagnetic wave effect, and power cannot be completely transferred. This phenomenon can be effectively reduced or eliminated by using the impedance match circuit. The conventional impedance match circuit has many disadvantages such as having too many elements and being of low precision. The redundant elements result in increased circuit area and unnecessary power consumption.
U.S. Pat. No. 6,560,290 discloses a complementary metal oxide semiconductor driver and a chip impedance match circuit for high-speed data communication. FIG. 1 is a circuit diagram of this impedance match circuit. In FIG. 1, resistors R11, R12, R13, and R14 are connected in series to divide the voltage of voltage source VCC and output it to operational amplifiers OP11, OP12, OP13. The operational amplifiers OP11, OP12, and OP13 connect two ends of the transistors 101, 106, and 109, and drive the operational amplifiers OP11, OP12, and OP13 by inputting voltage. The transistors 101, 104, 106, and 109 and the corresponding transistors 102 and 103, 105, 107 and 108, 110 and 111 form current mirrors. The source and drain of the transistor 101 are conducted, thus generating a current flowing through an applied resister R1e. The current is mirrored to the transistors 102, 103, and the current flowing through the transistors 102, 103 further flows through the transistors 104, 106 respectively and then is mirrored respectively to the transistor 105 and the transistors 107 and 108. The current flowing through the transistor 105 is mirrored to the transistors 110, 111 via the transistor 109. Thus, the equivalent impedance of the receiving ends RX+ and RX− is the impedance of the applied resistor R1e. 
The conventional circuit has a disadvantage that too many operational amplifiers result in a complicated layout and sensitivity to offset voltage, and thus it is difficult to achieve consistency of manufacturing processes. Meanwhile, as the equivalent resistance is simulated by a transistor, the channel modulation effect and the substrate parasitic effect are likely to occur, such that the high-frequency effect is limited.
ROC Patent Publication No. 538602 discloses another conventional impedance match circuit. FIGS. 2A and 2B are circuit diagrams of this conventional impedance circuit. An operational amplifier OP20 receives a reference voltage VB to conduct the transistor 201, so as to generate a current (with a current value of VB/R2e) flowing through an external resistor R2e. The transistor 202 mirrors the current to the transistors 203_1˜203_n. The resistor R22 is controlled by a switch SW to form a different resistor group together with the resistor R21. Taking the resistor R21 connected to the transistor 203_3 and two resistors R22 as an example, the resistor R21 is connected parallel with two resistors R22 via two switches SW. In the way of parallel connection, the equivalent impedance of this group drops due to the parallel connected resistors R22, thus generating a low voltage difference that is compared with voltage VB by the operational amplifier OP21_3. Other resistor groups may be deduced by analogy. After comparing different voltages with voltage VB, the operational amplifiers OP21_1˜OP21_n output signals T21_1˜T21_n to control the switches SW21_1˜SW21_n as shown in FIG. 2B. For example, when the signals T21_1˜T21 _2 are 1 and T21_3˜T21_n are 0, the impedance value of the resistor R2e is regarded as a value most approximate to the equivalent resistance of the resistor R21 connected parallel with a resistor R22. At this moment, the switch SW21_1 is on, such that the resistor R23 and one resistor R24 generate the desired match resistance value at output terminals VX and VY. The disadvantage of this conventional circuit lies in that the number of elements is large, resulting in increased circuit size.
ROC Patent Publication No. 538602 further discloses a conventional impedance match circuit. FIGS. 3A and 3B are circuit diagrams thereof. An operational amplifier OP30 receives a reference voltage VB that is input to the transistor 301, thus generate a current flowing through the external resistor R3e. The width-length ratio of the transistor 305 and 309 is p:q, and any width-length ratio of the transistor 305 and 307 is p:1. A control timing generator 350 turns on the switches SW31_1˜SW31_n in sequence, such that the ratio of the current flowing though the resistor R3e to the current flowing through the resistor RO is p+x/q (x is the number of ON-state switch SW31). The current generated by the transistor 309 generates a voltage drop at the resistor RO, i.e., a voltage VO is generated at the position where the operational amplifier OP31 is connected. The operational amplifier OP31 compares voltage VB with voltage VO, and stores the compared result to the register units 321_0˜321_n. The output signals TS_0˜TS_n of the register units 321_0˜321_n can control the switches SW32_0˜SW32_n of FIG. 3B, and thus the resistor R33 and the resistor R34 generate a match resistance value most related to the resistor R3e at the output terminals VX and VY. Although this conventional circuit is relatively simple, the mechanism for comparing voltages is still complicated, and the precision may be reduced when matched with transistors.